Computerized images are commonly printed on products such as paper and garments. Such printing is often performed using ink or toner jet printers. An example of a commercial computer-to-textile inkjet printing system is the 93X Series of CMYK digital printers from Kornit Digital Ltd., which is described in U.S. patent publication US2005/0179708 entitled “Digital Printing Machine”, and is hereby incorporated by reference herein for all that it teaches. Systems using ink jet printing technology involve placing the item to be printed on a printing table and holding the item in the proper position by means of a frame having an opening allowing the ink to be applied. The printing operation is performed while relative motion occurs in two axes. The printing table assembly bearing the item moves along a first axis, referred to herein as the “scanning” direction, while an assembly containing an array of printing heads, each having multiple inkjet nozzles, moves perpendicularly to the scanning direction, referred to herein as the “sub-scanning” direction.
Print heads employed in inkjet printers and the like usually each contain a plurality of nozzles arranged in (an) array(s). The nozzles usually are placed substantially equidistant. The distance between two contiguous nozzles defines the “nozzle pitch”. In operation, the nozzles are controlled to image-wise discharge fluid droplets of a marking substance on an image-receiving medium. When the printer is of the scanning type, the print heads are movable in reciprocation across the image-receiving medium (along the main scanning axis). In such printers, the print heads are typically aligned along a sub-scanning axis that is perpendicular to the direction of the main scanning axis. In a traverse of the print heads across the image-receiving medium a matrix of image dots of a marking substance, corresponding to a part of an original image is formed on the image-receiving medium by image-wise activating nozzles of the print heads. The printed matrix is generally referred to as a “print swath”, while the dimension of this matrix along the sub-scanning axis is referred to as the “swath width”. After a first traverse, when a part of the image is completed, the image-receiving medium is displaced relative to the print heads along the sub-scanning axis enabling printing of a subsequent part of the image. When this displacement step is chosen equal to a swath width, an image can be printed in multiple non-overlapping swaths. However, image quality may be improved by employing printing devices enabling the use of multiple printing stages; hence printed swaths are at least partially overlapping. In the background art, two main categories of such printing devices can be distinguished, i.e. so-called “interlace systems” and “multi-pass systems”.
In an interlace system, the print head contains N nozzles, which are arranged in (a) linear array(s) such that the nozzle pitch is an integer multiple of the printing pitch. Multiple printing stages, or so-called interlacing printing steps, are required to generate a complete image or image part. The print head and the image-receiving medium are controlled such that in M printing stages, M being defined here as the nozzle pitch divided by the printing pitch, a complete image part is formed on the image-receiving medium. After each printing stage, the image-receiving medium is displaced over a distance of M times the printing pitch. Such a system is of particular interest because it achieves a higher print resolution with a limited nozzle resolution.
In a “multi-pass system”, the print head is controlled such that only the nozzles corresponding to selected pixels of the image to be reproduced are image-wise activated. As a result, an incomplete matrix of image dots is formed in a single printing stage or pass, i.e. one traverse of the print heads across the image-receiving medium. Multiple printing stages are required to complete the matrix of image dots. The image-receiving medium may be displaced along the sub-scanning axis in-between two passes.
The amount of time required to print an image depends on several factors, including at least the print resolution of the image to be printed and the size of the printed image. With regard to the print resolution, generally the nozzle pitch (defined as the distance between centers of two adjacent nozzles) is greater than the printing pitch (defined as the distance between centers of two contiguous dots of ink both along the main scanning axis and along the sub-scanning axis) at the desired print resolution, and hence several printing stages per swath are required. As referred to herein, a “print pass” is defined as the large step movement of the print head(s) to print one complete swath of the image at the desired resolution. In contrast, a “print stage” is defined as the small step movement of the print head(s) relative to the image receiving medium to perform a single scan and deposition of ink within a single swath. Accordingly, one or more print stages may be required per print pass to achieve the desired resolution.
With regard to the size of the printed image, images that are wider than a single swath along the sub-scanning axis are printed as multiple adjacent (over-lapping or non-overlapping) swaths. Since each swath is printed in one print pass, images that are wider along the sub-scanning axis must be printed as multiple swaths and require correspondingly multiple print passes. It will be evident that the more print passes required to print an image, the longer the total print time. The number of items that can be produced by a printing system during a given period of time is, of course, directly affected by the time required to print each item. If the time required to print images on at least some items could be reduced, the number of items that could be produced with the system during a period of time would increase accordingly.